1. Field of the Invention
The invention relates to optical signal processing including, but not limited to, switching, amplification, correlation, combinational logic and algorithmic processing, optical wavelength discrimination, optical signal generation, and optical compensation. Such signal processing is preferably performed solely within the optical domain.
2. Description of the Prior Art
Present day optical signal processing is performed by a diverse variety of elements specifically dedicated to the functions to be performed and spanning a wide range of design concepts and technologies.
Optical amplifiers for fiber optic systems have been constructed utilizing Erbium-doped fiber that is optically pumped by semiconductor diode lasers. Information signal pulses passing through an optically pumped, Erbium-doped region, cause stimulated emission adding photons to the pulses resulting in optical amplification thereof. Other known technologies for providing optical amplification comprise the semiconductor laser amplifier (SLA) and the Raman amplifier. The SLA is exemplified by the Fabry-Perot SLA or the traveling-wave SLA. The described optical amplifiers function because an electronic population is inverted in a material such as glass or semiconductor. The Fabry-Perot SLA inverts the population inside a semiconductor by an electrical field.
Although the Erbium amplifier is in present day use for long-haul fiber optic communication systems, the amplifier tends to be very expensive. The Fabry-Perot SLA is also extremely expensive, on the order of $20,000.00 each in limited production quantities. The Fabry-Perot SLA is not an all-optical device, which may limit the use thereof in remote locations. The prior art optical amplifiers described may also be bandwidth limited because of the finite time required for the system to repopulate for the next arriving pulse. In such amplifiers, the light is propagated without the possibility of conversion, such as frequency conversion, or deflection from one path to another. In such amplifiers the light signal remains in its original path. Such amplifiers are like pipelines where optical signals pass from one waveguide to another without conversion or bending. The optical signals are amplified within the amplifier waveguide. The traveling-wave SLA and the Raman amplifier are of theoretical interest and have not enjoyed widespread commercial usage. This is because of manufacturing difficulties and optical losses associated with transfer of the optical power to the waveguide and then back to a fiber. Other difficulties are associated with attaining sufficient gain in the amplifier cavity while suffering attenuation therein.
In present day fiber optic systems, mechanically actuated optical switches are primarily utilized. Such mechanical switches function by movement of a directing mirror through activation by a solenoid and tend to have short operational lifetimes because of mechanical wear-out through repeated switching. Magneto-optical and electro-optical switching are also known, although not in widespread usage. Electro-optical switches of many types have been demonstrated as integrated optical devices. Waveguide loss in the integrated optics devices remains high. It is difficult to get light into integrated optical devices without appreciable loss of signal strength. Magnetic or electrical fields generated by electrical voltages and currents provide the light switching actuation. Since such switches are not all optical, usage thereof at remote locations may be limited. In addition, the electrical voltages and currents tend to generate electronic and magnetic interference and noise in adjacent equipment. Such switches also tend to be less reliable than all-optical devices because of potential failures in the electrical and magnetic driving circuits and equipment.
Optical signal computation and processing presently remains an unfulfilled objective. Optical signal processing is desirable because of the significantly greater speeds potentially available from optical devices and because of the relatively low power consumption required thereby. An all-fiber optical processor would be desirable because of the facility with which optical signals are transported through optical fibers. Such processors may perform functions including power spectrum decomposition, matrix inversion, Eigenvalue solutions, weighted sums and other digital algorithmic functions. Presently, optical computing has only been demonstrated in bulk optical forms such as acoustic signal processors, spatial light modulators, and integrated optics devices. The difficulty of using bulk optical components are requirements of a vibration free table, complex optical alignments, large system loss, and difficulties in coupling light into bulk devices. Spatial light modulators are lossy and require spatial division and transfer of many separate optical beams. They are generally slow and require optical tables, benches, and the like. Integrated optical devices have fairly high loss and are, presently, difficult to mass produce. Fiber attaches to the input of integrated optical devices with high loss due to mismatch of modal light fields, Fresnel reflection, and temperature induced instabilities due to thermal coefficient of expansion mismatch. All of these devices may be attached or strung together with fiber but tend to be lossy. It is believed that substantially no all-fiber optical processors have been demonstrated to date.
In an unrelated technology, the variable ratio fiber optic coupler sensor is known and described in U.S. Pat. No. 4,634,858, issued Jan. 6, 1987, entitled "Variable Coupler Fiberoptic Sensor" by Gerdt et al. Said U.S. Pat. No. 4,634,858 is incorporated herein by reference. Said U.S. Pat. No. 4,634,858 describes the sensor as a device that varies the output coupling ratio in accordance with stress induced in a photoelastic encapsulating material. The index of refraction of the encapsulating material is described as varying with stress applied thereto, and the output coupling ratio of the sensor is described, in the patent and elsewhere, as a highly sensitive function of the index of refraction. The sensor is utilized to sense acoustic, electric, magnetic and mechanical fields. Any mechanism that converts a physical or field effect into a force that applies stress to the encapsulating material may be sensed by the device. The fiber optic coupler sensor may be considered as an optical strain gauge, whereby strain resulting from a parameter to be measured results in a modulation or change in the coupling output ratio of the coupler sensor in accordance with the magnitude of the parameter to be measured. Parameters which do not generate strain cannot be sensed by a strain gauge, whether optical or based on a non-optical technology. Thus, variations in light intensity that do not result in stress applied to the encapsulating material are not sensed by the device. It is believed that these devices can transfer power between output fibers because of bending which occurs in the fused waist region.